Yes you can add additional groups and bones to a conforming item. A lot of rigged hair models, dresses, long coats, etc., are rigged that way.

Your model just needs to have groups that correspond to the figure's groups that it's being made for. There is no universal rigging convention in Poser. There's a loose standard that most figures adhere to but all of them have variations on that based on whatever its creator decided. The important point is that conforming items made for that figure need to match. They can have additional parts, but they still need to have the parts that line up with the figure's rigging.

Hip is the most commonly used name for a figure's root bone.

Python can be used to control and modify most everything inside Poser but it's not necessary for rigging figures.

There's no real repository for info regarding the inner workings of Poser files. There's a (very outdated) book called Secrets of Poser but it's nearly 20 years old and out of print. Last time I saw a copy for sale online it was over $200. You can find sample chapters of it in google but I've seen various bits that aren't relevant to Poser's current iteration. (Example: you don't need to edit any code in a file to create a smart prop. Just parent it to whatever you need and save it to the library as a smart prop).

A lot of info has been lost over the years due to major forums closing like rDNA being consumed by Daz, and Smith-Micro closing their forums when they sold Poser to Bondware. So the biggest source of info for everything Poser-related currently is the renderosity forum archives.

Just ask questions here in the forums, or search the archives.

Also, you might not want to post your email in the forum unless you don't mind it being picked up by bots.

ETA: One thing to note, in case you aren't clear on it, is that grouping and rigging is only for conforming items. You don't group or rig dynamic clothing meant for the cloth room - it will fall apart at the seams. There is grouping used in dynamic cloth but that controls how the cloth behaves in simulations and is completely different than conformers.